echoesof.science · Ages 15–16
Sensory receptors: transduction, receptive fields and thresholds
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The familiar five senses can be divided into as many as 17 modalities. Receptors are classified by structure (free nerve endings, encapsulated endings, specialised receptor cells), by location (exteroceptors, interoceptors, proprioceptors) and by the stimulus they respond to (chemo-, osmo-, noci-, mechano-, thermo- and photoreceptors).
A receptor transduces mechanical, thermal or chemical energy into electrical signals that travel along axons to the central nervous system. Slowly adapting receptors keep firing during a constant stimulus; rapidly adapting receptors fire mainly when a stimulus starts or stops.
Skin has several mechanoreceptors: Merkel discs respond to steady pressure and low-frequency vibration (about 5–15 Hz), Meissner's corpuscles to light touch and vibration below 50 Hz, Pacinian corpuscles to deep pressure and high-frequency vibration (around 250 Hz) and Ruffini corpuscles to stretch. Free nerve endings detect pain and temperature.
Experiment: Two-point thresholds and receptive fields
- Work in pairs. The subject sits with eyes closed. Test four sites: fingertip, palm, back of the hand and forearm.
- Set the two ends 40 mm apart. Touch the skin gently, with both ends at the same moment and the same light pressure. Randomly mix in touches with only one end, so the subject cannot guess.
- The subject says 'one' or 'two'. Reduce the gap in 5 mm steps, then in 1 mm steps near the point where 'two' stops being reported.
- Define the threshold as the smallest gap at which 'two' is reported correctly in at least 3 of 4 two-point touches. Repeat the whole search 3 times per site.
- Calculate the mean threshold and the uncertainty (half the range) for each site. Calculate the ratio forearm ÷ fingertip and compare with the textbook values (about 2–3 mm and 35–40 mm).
| Site | Run 1 (mm) | Run 2 (mm) | Run 3 (mm) | Mean ± uncertainty (mm) |
|---|---|---|---|---|
| Fingertip | ||||
| Palm | ||||
| Back of hand | ||||
| Forearm |
Questions
Classify the Pacinian corpuscle in three ways (structure, location, stimulus).
Use the idea of receptive fields to explain why the fingertip threshold is much smaller than the forearm threshold.
Give your mean thresholds with uncertainties for the fingertip and forearm and calculate their ratio. Do they overlap with the textbook values?
Explain, using the route smells take while chewing, why food tastes bland during a cold.
Sources for every fact on this sheet: echoesof.science/school/senses/sources/